The invention relates to a thermally tripped circuit breaker switch which operates according to the principle of thermal tripping by means of a bimetal strip.
A switch of this type is disclosed, for example, in German Patent No. 2,502,579 and corresponding U.S. Pat. No. 4,044,325. The bimetal strip is here given an approximately right-angle bend and is supported with little play near the bend by two bead projections shaped to a switch housing. At an appropriate distance next to the bead projections, a contact lug is shaped to the bimetal strip so as to project laterally in the direction toward a contact piece of a right-angle contact bridge. In a turn-on position of the circuit breaker switch, a tripping spring urges the contact piece of the contact bridge against the contact lug. If an overcurrent occurs, the bimetal strip is heated to the extent that it is bent outwardly to release the contact bridge from the contact lug which is pushed back into its turn-off position by the force of a compression spring.
The tripping sensitivity of the switch is adjusted by way of an adjustment screw. The adjustment screw acts according to the principle of pull-push adjustment by way of a bias and deformation on a connecting arm fixed to the bimetal strip. The drawback of this adjustment is the lability of the adjustment screw in the end state of the adjustment. The biasing force acting on the screw changes over the adjustment range. Moreover, the biasing force acts only on the point in the center of the adjustment screw core.
This lability is disadvantageous because such circuit breakers are preferably installed as substitutes for conventional meltable fuses in the circuits of vehicles. Due to vehicle operation, the switch is subjected to varied use conditions. The switch must be able to withstand abrupt changes in climate and also extreme mechanical shock and vibration stresses. Particularly, if the biasing forces are low, such adjustment will not be able to handle these climatic and mechanical stresses.
Moreover, very little space is available for the installation of circuit breakers in vehicles, so that plug bases are provided into which such switches are inserted. For this purpose, the switches are provided with contact elements in the form of knife blade lugs. In order to be able to place as many as possible of such switches in juxtaposition in the closest space in a plug base, the individual switch must be configured as a very narrow structure. For reasons of space economy, the depth of the switch must be kept as low as possible for the same reason. The height of the switch, however, permits a little more leeway.
In the required, successively miniaturized configuration, these knife blade lugs lie very close to one another so that the space available between them for an adjustment screw is minimal. The adjustment screw itself, however, must not fall below certain dimensions, since it must be designed to handle the active biasing forces. Moreover, an external manipulation of the adjustment screw by, for example, a screwdriver, must be ensured. The size of the adjustment screw is thus a significant parameter which limits further miniaturization of such a switch.
Another drawback is that such adjustment screws are manufactured of metal and thus of conductive materials and, if they are placed between closely adjacent knife blade lugs, the danger of short circuits increases. The risk of damage to the knife blade lugs by the adjustment screw screwdriver also exists.
U.S. Pat. No. 3,265,832 discloses a bimetal controlled circuit breaker which is adjusted by way of an adjustment cam which charges the bimetal strip. For this purpose, the adjustment cam is externally adjustable. The drawback of this circuit breaker is the complicated overall structure composed of many individual parts.